Wrist motion is the result of interaction of carpal kinematics occurring at the different levels of this complex and composite joint. Forces across the wrist are transmitted following variable patterns depending upon internal musculotendinous contraction, externally applied forces, and distal forearm and carpal bone relative orientation. Strong clinical evidence suggests that carpal instabilities affect the global motion and functional strength of the wrist. Comprehensive biomechanical analyses of these changes should provide the essential knowledge and tools needed to grade the pathologic involvement while different treatment modalities associated with these injuries could be compared objectively to rationalize their roles in patient care. The goal of this project is to increase the knowledge about the normal function of the wrist, as well as the pathomechanical effects of several types of carpal instability on: (1) carpal kinematics; (2) intercarpal and radioulnocarpal contact areas; (3) load transmission across the wrist and the associated virtual displacement between adjacent carpal bones; and (4) carpal ligamentous tension. Pathological conditions will include: (1) unstable scaphoid fractures; (2) dissociative carpal instabilities (scapho-lunate and luno-triquetral); and (3) non-dissociative carpal instabilities (VISI types). Carpal kinematics and ligament strains will be analyzed by a biplanar roentgenographic method using marked cadaver specimens. Joint contact area will be quantitated by reconstructing CT scan images of wrist injected with interarticular contrast media. Three-dimensional load transmission across the wrist will be performed using the rigid-body-spring model(RBSM) to determine axial force, articulating surface pressures, ligament tensions and intercarpal bone virtual (attempted) displacements under different external forces and wrist positions. The shape of the carpal bones will be obtained from the CT data and the elastic properties of f the cartilage and carpal ligaments will be studied using the established indentation creep and tension tests. The axial force transmitted from scaphoid and lunate to the distal radius and ulnar fossae will be validated by a custom-made loading frame with three multi-directional load cells fixed to the lateral, central and ulnar columns of the distal forearm. Finally, patients with chronic wrist instabilities will be evaluated functionally using triaxial electrogoniometer and wrist dynamometer under isometric mode while hand grip forces will be monitored simultaneously. These results will be correlated with radiographic and clinical assessments.